Testosterone (T) supplementation increases muscle mass and decreases fat mass in a variety of clinical trials using healthy young men, hypogonadal men, and in older men with low serum T levels. However, the molecular mechanisms by which T regulates body composition are not well known. We identified that T increases muscle mass and decreases fat mass in mesenchymal multipotent cells via androgen receptor (AR)-mediated pathway and activates AR/beta-catenin/TCF-4 pathway in preadipocyte cells to inhibit adipogenesis that was associated with up-regulation of follistatin (Fst). We hypothesize that T activates AR/beta-catenin/TCF-4 pathway both in vitro and in vivo to activate Fst, which regulates body composition by inhibiting Mst/TGF-beta signaling. We will employ two validated in vitro models of adult, mesenchymal stem cells to test whether T and DHT activate Fst via activation of AR/beta-catenin/TCF-4 pathway to regulate cell differention, and inhibition of beta-catenin by SiRNA abolishes these effects. We will determine the interaction between AR, beta-catenin, and TCF-4 by immunoprecipitation and their nuclear translocation by immunofluorescence and cell fractionation. TCF-4 transcriptional activation in response to androgen will be measured by using TCF-4 luciferase reporter assay. We will determine whether these androgens inhibit TGF-beta/Mst signaling in vitro through up-regulation of Fst. We will test whether T and DHT inhibit Mst bioactivity, Smad2/3 phosphorylation and activate Smad7 in vitro and inhibition of Fst levels either by Fst SiRNA or use of anti-Fst antibody blocks the effects of these androgens on myogenic differentiation. We will investigate whether castration-induced effects on body composition in C57/BL6J mice are accompanied by parallel decrease in Fst levels and supplementation of T or recombinant follistatin in these castrated mice significantly block castration-induced effects on fat-free mass and overall body composition. These studies will provide novel insights into the mechanisms of androgen action and have direct relevance to the application of either Fst alone or in combination of low doses of T for the treatment of HIV and aging-associated muscle loss as high supraphysiological doses of T are associated with its pleotropic effects including increased risk of prostate cancer by elevating prostate-specific antigen.